1. Field of the Invention
The present invention relates to rare earth (RE) mixed phosphates especially useful for the production of phosphor materials, and, more particularly to cerium lanthanum terbium mixed phosphates especially useful as green phosphor precursors, as well as to a process for the production of such precursors.
2. Description of the Prior Art
Since 1970 it has been known to this art that rare earth mixed phosphates and especially those of cerium, lanthanum and terbium exhibit an advantageous luminescence property. Thus, numerous cerium lanthanum terbium phosphates, also designated "LaCeTb phosphates" have been developed with different concentrations of lanthanum, cerium and terbium, as have various processes for the production thereof.
Such processes for the production thereof can be divided into two basic categories, namely, processes which entail a "dry route", and processes which entail a "wet route".
The dry-route processes, described especially in JP 62/007,785, WO 82/04,438, JP 62/089,790, JP 59/179,578 and JP 62/000,579 include forming a mixture of rare earth oxides or phosphating a rare earth mixed oxide by calcination in the presence of diammonium phosphate.
The "wet-route" processes, described especially in JP 57/023,674, JP 60/090,287 and JP 62/218,477, entail a direct synthesis of a rare earth mixed phosphate or of a mixture of rare earth phosphates by digestion of a solid compound (carbonate, oxide) with H.sub.3 PO.sub.4 to precipitate the phosphates.
U.S. Pat. No. 3,507,804 describes a process for the production of lanthanum terbium double phosphate by precipitating the phosphate from a solution of rare earth nitrates and adding phosphoric acid thereto. However, the phosphate or phosphates obtained are very difficult to filter.
These different processes produce mixed phosphates requiring, for their application in luminescence, a heat treatment at a high temperature, approximately 1,200.degree. C., under a reducing atmosphere. Indeed, in order that the LaCeTb phosphate provide a green phosphor, the cerium and the terbium must be in the 3+ oxidation state.
In addition, the efficiency of the phosphatation must be close to 100% in order to obtain a phosphor which is as pure as possible and to obtain a maximum emission efficiency, and this requires numerous precautions and a relatively long treatment in the case of the "dry-route" processes.